lifeboot.asm

   1 bits 16
   2 org 0x7c00
   3
   4 %define ORG 0x7c00
   5 %define VGA 0xb8000
   6
   7 %define COLS 80
   8 %define ROWS 25
   9
  10 %define DEAD ' '            ; char to represent a dead cell
  11 %define ALIVE '#'           ; char to represent an alive cell
  12 %define PRINT_COLOR 0x07    ; grey on black
  13
  14 %define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
  15 %define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
  16 %define XSS         NEXT_GRID + COLS * ROWS ; xs() state
  17
  18 ; entry point -----------------------------------------------------------------
  19
  20 ; initialize segment registers
  21 xor ax, ax
  22 mov ds, ax
  23 mov es, ax
  24 mov ss, ax
  25
  26 ; set stack pointers
  27 mov sp, ORG
  28 mov bp, ORG
  29
  30 ; clear direction flag
  31 cld
  32
  33 ; disable cursor
  34 mov ch, 0x3f
  35 mov ah, 0x01
  36 int 0x10
  37
  38 ; initialize xss
  39 mov ah, 0x00                ; get
  40 int 0x1a                    ;  system time
  41 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
  42
  43 ; setup -----------------------------------------------------------------------
  44
  45 setup:
  46
  47 call init_grid
  48 call print_grid
  49
  50 jmp halt
  51
  52 ; functions -------------------------------------------------------------------
  53
  54 ; xs() - xorshift pseudorandom number generator -------------------------------
  55
  56 ; output:
  57 ; [XSS]     = updated state
  58
  59 ; clobbers bx, dx
  60
  61 xs:
  62
  63 mov bx, [XSS]
  64 mov dx, bx
  65
  66 shl dx, 1                   ; dx = xss << 1
  67 xor bx, dx                  ; bx = xss ^ (xss << 1)
  68 mov dx, bx
  69
  70 shr dx, 3                   ; dx = xss' >> 3
  71 xor bx, dx                  ; bx = xss' ^ (xss' >> 3)
  72 mov dx, bx
  73
  74 shl dx, 10                  ; dx = xss'' << 10
  75 xor bx, dx                  ; bx = xss'' ^ (xss'' << 10)
  76 mov [XSS], bx
  77
  78 ret
  79
  80 ; init_grid() - initialize GRID cells with random values ----------------------
  81
  82 ; (*) expects es = 0.
  83
  84 ; clobbers ax, bx, cx, dx, di
  85
  86 init_grid:
  87
  88 mov di, GRID                ; es:di -> GRID
  89 mov cx, COLS * ROWS         ; for each cell
  90
  91 .write_cell:                ; do {
  92     call xs                 ;     [XSS] = rand
  93     mov al, DEAD            ;     al = DEAD (likely)
  94
  95     test word [XSS], 0b11
  96
  97     jnz  .nz                ;     if ([XSS] % 4 == 0)
  98     mov al, ALIVE           ;         al = ALIVE
  99
 100 .nz:
 101     stosb                   ;     [es:(di++)] = al
 102
 103     loop .write_cell        ; } while (--cx)
 104
 105 ret
 106
 107 ; print_grid() - print GRID cells to screen -----------------------------------
 108
 109 ; (*) this function expects ds = 0 when called and sets es = 0 before
 110 ;     returning. this is simpler because in the rest of the program we want the
 111 ;     segment registers to be zeroed anyway.
 112
 113 ; clobbers ax, cx, di, si
 114
 115 print_grid:
 116
 117 mov ax, VGA >> 4
 118 xor di, di
 119
 120 mov es, ax                  ; es:di -> VGA
 121 mov si, GRID                ; ds:si -> GRID
 122
 123 mov ah, PRINT_COLOR         ; ah = color attribute
 124
 125 mov cx, COLS * ROWS         ; for each cell
 126
 127 .print_cell:                ; do {
 128
 129     lodsb                   ;     al = [ds:(si++)]
 130     stosw                   ;     [es:di] = ax ; di += 2
 131
 132     loop .print_cell        ; } while (--cx)
 133
 134 mov es, cx                  ; es = 0
 135 ret
 136
 137 ; halt() - stop program execution ---------------------------------------------
 138
 139 halt:
 140
 141 cli                         ; disable interrupts
 142 hlt
 143 jmp halt
 144
 145 ; -----------------------------------------------------------------------------
 146
 147 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 148 dw 0xaa55                   ; mbr magic byte